1. Field of the Invention
This invention relates to digital voice transmission systems and, more particularly, to a new technique for increasing the signal-to-noise ratio (SNR) in a code excited linear predictive (CELP) speech coder.
2. Description of the Prior Art
An early description of CELP coding was published by M. R. Schroeder and B. S. Atal in "Stochastic Coding of Speech Signals at Very Low Bit Rates", Proc. of 1984 IEEE Int. Conf. on Communications", May 1984, pp. 1610-1613, although a better description can be found in M. R. Schroeder and B. S. Atal, "Code-Excited Linear Prediction (CELP): High-Quality Speech at Very Low Bit Rates", Proc. of 1985 IEEE Int. Conf. on Acoustics, Speech, and Signal Processing, March 1985, pp. 937-940. The basic technique comprises searching a codebook of randomly distributed excitation vectors for the vector that produces an output sequence (when filtered through pitch and linear predictive coding (LPC) short-term synthesis filters) that is closest to the input sequence. To accomplish this task, all of the candidate excitation vectors in the codebook must be filtered with both the pitch and LPC synthesis filters to produce a candidate output sequence that can then be compared to the input sequence. This makes CELP a very computationally-intensive algorithm, with typical codebooks consisting of 1024 entries, each 40 samples long. In addition, a perceptual error weighting filter is usually employed, which adds to the computational load. A block diagram of a known implementation of the CELP algorithm is shown in FIG. 1, and FIG. 2 shows some example waveforms illustrating operation of the CELP method.